An important class of devices for telecommunications applications are wavelength tunable laser transmitters. Because of their aging characteristics these laser transmitters can shift in wavelength over time for a given set of fixed operating parameters. In order to correct for this shift in wavelength, some means of wavelength stabilization is typically employed. One type of wavelength stabilizer, or ‘wavelocker’ relies on the spectral transmittance of a Fabry-Perot etalon. These etalons are designed such that the transmitted spectrum is repeated at regular frequency intervals (ie the channel spacing). The transmission of a portion of the laser light through these devices gives a measure of the laser's wavelength since the transmitted spectrum of these devices is stable over time. One problem with these devices is that the spectrum is, in general, not fixed as the operating temperature changes. The temperature shift in these etalons arises from a change in the optical path length with temperature, that is:                                           1            nL                    ⁢                                    ⅆ                              (                nL                )                                                    ⅆ              T                                      =                  α          +                                    1              n                        ⁢                                          ⅆ                n                                            ⅆ                T                                                                        (        1        )            
where α is the coefficient of thermal expansion, L is the length of the etalon, n is the index of refraction of the etalon and the product, nL, is the optical path length. An etalon could be made athermal if a material could be found such that the right hand side of the above equation equals zero. Unfortunately, at this time, we do not know of any such material.
Fabry Perot etalons and Gires Tournois etalons are used as building blocks in a variety of optical devices to provide functionality to a host of solutions to applications in the optical domain. By way of example, U.S. Pat. No. 5,798,859 in the name of Colbourne et al., discloses a wavelength locker circuit, wherein an element having a wavelength dependent characteristic such as a Fabry Perot etalon is used to provide an output signal having an amplitude that varies with wavelength. The intensity of a reference signal derived from an input signal is compared with an output from the Fabry Perot etalon to provide a feedback signal that corresponds to the frequency of the input signal. Another wavelocker circuit that utilizes an etalon is disclosed in U.S. Pat. No. 6,560,252 in the name of Colbourne et al.
As with most optical circuitry, changes in temperature typically adversely affect the performance of a device. For example, the optical path length and free spectral range changes in an etalon as temperature varies. In the majority of instances this resulting change in output response with a temperature change is unwanted and deleterious to the operation of the device. It is generally assumed and desired, for most optical devices to operate in a stable manner with drifting temperature. When two different etalons are required, having different optical path lengths, and their output responses are to be combined in some manner, differences in their individual responses due to changes in temperature can be highly problematic compromising the functionality of such a system. By way of example, this can also occur in optical devices such as Mach Zehnder interferometers, where a fixed, predetermined, or predictable optical path length difference in two arms is required and must be maintained. One solution is to provide a temperature compensation circuit, which is costly to implement, and requires power to heat or cool in order to maintain an optical circuit or component at a constant temperature.
It is an object of this invention, to provide an interferometric device that is substantially athermal; that is, being substantially unaffected by temperature changes at normal operating temperatures.
It is a further object of this invention, to provide a method for selecting lengths of two arms of an interferometer, given two materials, their refractive indices and coefficients of thermal expansion, that will yield a substantially athermal structure.
It is a further object of this invention to provide a substantially athermal interferometer that does not require an active temperature compensation circuit.